In recent years, there has been a demand for conversion of seawater into freshwater by desalination in large coastal cities in arid or semi-arid regions where it is difficult to constantly secure water resources. For the seawater desalination, evaporation techniques implemented in large-scale plants in high-temperature regions, and membrane techniques using a reverse osmosis membrane for desalination, have been commercialized. In recent years, seawater desalination using membrane techniques has been drawing attention by virtue of its high desalination efficiency and the like.
For such seawater desalination by membrane techniques, the following methods have been proposed: a method aimed at preventing treatment efficiency from decreasing due to entrance of contaminants such as polymers into a reverse osmosis membrane, the method consisting of treating seawater with an ultrafiltration membrane or a low-pressure reverse osmosis membrane, and then desalting the seawater using a reverse osmosis membrane; and a method aimed at improving energy efficiency by decreasing the salt concentration of seawater entering a reverse osmosis membrane, the method consisting of mixing seawater with river water or membrane bioreactor-treated water, and then desalting the seawater using a reverse osmosis membrane (see Patent Literature 1, for example). However, with these conventional methods, it is not possible to suppress occurrence of biofouling which is caused, for example, on the surface of a reverse osmosis membrane by microorganisms and the like. Accordingly, there is observed a tendency that the permeation performance or the separation performance of the reverse osmosis membrane is significantly deteriorated as a result of the reverse osmosis membrane being used continuously for long years. Meanwhile, in water treatments including but not limited to seawater desalination, approaches to suppress biofouling by addition of bactericides or by an ultraviolet irradiation treatment have also been made (see Patent Literature 2, for example). However, it has been found that in the case of the methods mentioned above, the effect of an ultraviolet irradiation treatment is not sufficiently obtained because of high turbidity of the water to be treated, and the activity of microorganisms is consequently restored in the reverse osmosis membrane treatment.